1. Field of the Invention
The present invention relates to an optical pickup lens which is used in an optical system that performs recording or playback of an optical disc and, particularly, to a technique of detecting the tilt state of an optical pickup lens.
2. Description of Related Art
Various optical discs such as a compact disc (CD), a digital versatile disc (DVD) and a Blu-ray disc (BD) are developed and manufactured as media to be used for an optical disc drive. An optical system that performs recording or playback of such optical discs includes an optical pickup lens. The optical pickup lens plays an important role in focusing laser light on an optical disc when recording or playing back data on an optical disc.
Optical discs are produced so as to show a jitter value that is best suited to respective standards. However, optical discs cannot sufficiently exert their capabilities unless an optical pickup lens accurately focuses laser light on each optical disc and forms a suitable optical spot. Although an optical pickup lens can form a suitable optical spot by satisfying properties such as a shape, a refractive index and an aberration, even if an optical pickup lens that satisfies such properties can be manufactured, a suitable optical spot cannot be formed unless the optical pickup lens is accurately mounted to an optical pickup in an optical disc drive.
Generally, it is required to let the optical axes of laser light emitted from a laser and an optical pickup lens coincide with each other, and further to let the center of the opening in a lens holder of an optical pickup and the lens center of the optical pickup lens coincide with each other. The opening of the lens holder of the optical pickup is perpendicular to the optical axis of laser light. The opening surface of the lens holder is parallel to the mounting surface of the optical pickup lens.
One method of letting the opening center of the lens holder and the lens center of the optical pickup lens coincide with each other is to form an outer frame on the lens mounting surface in such a way that the outer flange of the optical pickup lens is fit into it, and adjust a difference in size between the outer frame and the optical pickup lens, which is “play”. According to this method, once the outer frame is determined, there is no possibility that any problem occurs when mounting the optical pickup lens to the pickup. Needless to say, in order to produce the outer frame into which the outer flange of the optical pickup lens is fit into on the lens mounting surface, it is necessary that the center of the outer diameter of the optical pickup lens and the center of the effective diameter of the optical pickup lens coincide with each other. This is the condition required to be satisfied when manufacturing the optical pickup lens.
A general method of letting the optical axes of laser light and an optical pickup lens coincide with each other is to apply laser light for lens tilt adjustment onto a flange surface outside the disc-side effective diameter of the optical pickup lens, measure the angle of reflected light of the laser light for lens tilt adjustment, adjust the tilt of the optical pickup lens and, fix the optical pickup lens by an adhesive or the like in the process of mounting the optical pickup lens. In some cases, it may be necessary to set the angle between the optical axis of the optical pickup lens and the optical axis of laser light to a fixed angle, rather than let them coincide with each other, in terms of manufacture or adjustment of the performance of the pickup. In such a case as well, it is common to employ the above-described method of using the laser light for lens tilt adjustment.
Preferable conditions for increasing the working efficiency in optical pickup lens tilt adjustment are that the amount of laser light for lens tilt adjustment reflected on the flange surface of the optical pickup lens, which is reflected light, is large, that the reflected light is sharp or well-defined, and that the optical spot of laser light for lens tilt adjustment focused on the flange surface has a suitable shape (e.g. non-distorted, perfect circle). An example of a method of achieving such conditions is to form a mirror-finished surface outside the effective diameter of the optical pickup lens and reflect the laser light for lens tilt adjustment on the mirror-finished surface. This method increases the amount of reflected light of the laser light for lens tilt adjustment and forms the optical spot of the laser light on the mirror-finished surface into a suitable shape.
In addition to the process of lens tilt adjustment, there is the process of applying laser light onto the area from the outside of the lens effective diameter to the lens outer diameter of the optical pickup lens and monitoring the state of a lens tilt. This is the process of adjusting the operating direction of the optical pickup to which the optical pickup lens is fixed, for example. In such a process as well, it is preferable to increase the amount of reflected light of laser light and to form a non-distorted circular optical spot on the position where the laser light is applied, just like the process of lens tilt adjustment.
An optical pickup lens for recording or playback of DVD discs and CD discs is generally made of a plastic material. Because the plastic material can be molded into any shape, it is possible to flexibly form the shape of the area from the outside of the lens effective diameter to the lens outer diameter of the optical pickup lens. It is thereby possible to form the areas from the outside of the lens effective diameter to the lens outer diameter on the laser side and the disc side of the optical pickup lens into different shapes from each other. Further, the lens effective diameter portion and the outside portion are often formed by different molds. Thus, by adjusting the relative positional relationship in the optical axis direction between a mold for the lens effective diameter portion and a mold for the outside portion to be fixed to the optical pickup, it is possible to adjust the distance between the laser of the optical pickup and the optical pickup lens to a certain degree. It is also possible to eliminate an unnecessary structure on the disc surface side.
In the optical pickup lens that is made of plastic and formed by molding, it is common to form a mirror-finished surface in a given position within the area from the lens effective diameter to the lens outer diameter on the disc surface side of the optical pickup lens.
On the other hand, an optical pickup lens for recording or playback of BD discs and HD-DVD discs is generally made of a glass material. In the case of using glass, the flexibility in the shape of the area from the lens effective diameter to the lens outer diameter is low. This is because if a complex shape is formed using glass, a difference is generated within the shape due to thermal shrinkage that occurs after press molding until the material is cooled to normal temperature, making it subject to breakage. For this reason, in the case of using a glass material, it is common to form the area from the lens effective diameter to the lens outer diameter into a flat shape. Further, in order to avoid a failure to accurately form the shape of the area of the lens effective diameter in the structure where the outer flange portion of the lens effective diameter is directly coupled to the flat area, the outer flange portion of the lens effective diameter may be coupled to the flat area via a curved portion in cross section.
In the optical pickup lens that is made of glass, the flat portions that are formed in the areas from the lens effective diameter to the lens outer diameter on the laser light source side and the disc side should be perpendicular to the lens optical axis in each of the laser light source side and the disc side; therefore, they are formed to be parallel to each other in most cases.
Japanese Unexamined Patent Application Publication No. 2003-222707 discloses an optical pickup lens whose surface is coated with an antireflection coating in order to enhance the light use efficiency. However, Japanese Unexamined Patent Application Publication No. 2003-222707 discloses nothing about the technique of applying laser light for tilt adjustment or the like.
In order to accurately mount an optical pickup lens onto an optical pickup and correctly detect the mounted state, there is a case of applying laser light from the disc side to the flange surface on the same side and reflecting the laser light not on the above flange surface but on the opposite surface (i.e. the surface on the side of laser for recording or playback). In such a case, however, the light that is reflected on the opposite surface serves as stray light that interferes with detection based on proper light.